Plasma membrane

 Plasma membrane or cell membranes

One of the component of a cell is plasma membrane, it acts like a shield. At the boundary of every cell, the plasma membrane functions as a selective barrier that allows passage of enough oxygen, nutrients and wastes to service the entire cell. In other words a cell membrane is a covering or boundary which bounds the cell and cytoplasm hence, called plasma membrane or cell membrane. Cellular membranes contain four components: (1) a phospholipid bilayer, (2) transmembrane proteins, (3) an internal protein network providing structural support, and (4) cell-surface markers composed of glycoproteins and glycolipids. The fluid mosaic model of membrane structure includes both the fluid nature of the membrane and the mosaic composition of proteins floating in the phospholipid bilayer It maintains cell structure, shape just like fencing around a garden or farm. In plant cell cell wall is another covering upon cell membrane made up of cellulose that's why plants are very stiff. Reason behind the importance of cell membrane and studying it deeply is the functioning of membrane, it surrounds the each cell and regulates its exchange with its environment and surrounding. Because of cell membrane many microbes like amoeba are able to take food (phagocytosis). Lets get a deep and visual look of cell membrane and it`s composition.

 Lipids and proteins are the staple ingredients of membranes, although carbohydrates are also important. The most abundant lipids in most membranes are phospholipid. A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic (water loving) region and a hydrophobic(water fearing) region. Picture shown below is the cross section of phospholipid bilayer along with molecule.

Cellular membrane are fluid mosaic of lipids and proteins

Like membrane lipids, most membrane proteins are amphipathic, it means that proteins are embedded in cell membrane, just like if you make a circle shape cut out of a cardboard and place a ball in that space. Such proteins can reside in the phospholipid bilayer with hydrophilic region protruding. This molecular orientation maximizes contact of hydrophilic regions in cytosol which is cytoplasm without organelles and extracellular fluid while also providing non aqueous environment in between. This picture shows the currently accepted model of the arrangement of molecules in plasma membrane fluid mosaic model, the membrane is mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. So, basically a mosaic is noting but a pattern of arrangement of cells or substances. By studying the molecule and all the substance which make up the cell is not complex or confusing, Just like us we protect and care our surrounding and our family if you are a good person, aren't you😕 just joking😁. Same is with the cell membrane it protects the cell, it does work by transporting, it is also a bit flexible also strong, it has some proteins and also some cholesterol. Here is a picture

Fluidity of cell membranes

Membranes are not static sheets of molecules locked rigidly in place. A membrane is held together mainly by hydrophobic interactions. In a membrane molecular movement happens due to which fluidity varies. Fluidity is nothing but it tells us how much the membrane is fluid or in other words how much watery it is. This fluidity occurs due to movement of molecules in membrane and the factors are presence of cholesterol, saturated and unsaturated hydrocarbons. Most of the lipids and some proteins can shift about sideways- that is in the plane of membrane, like making left and right movement .Very rarely also a lipid may flip flop across the membrane switching from one phospholipid layer to the other. Fluidity also depend upon temperature, as temperature decreases the membrane will remain as fluid until phospholipid molecule will get closely packed, just like if you freeze something it gets solidify. The particular temperature at which membrane get solidify depends on type of lipids it is made up of. Now unsaturated hydrocarbons make membrane more fluid because they form double bonds and also of kinks in tails which pushes adjacent molecule away, enhances membrane fluidity. Saturates hydrocarbons are closely packed and hence make membrane more viscous. The steroid cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipid movement, but at low temperature it hinders solidification by disrupting the regular packing of phospholipids. Membranes must be fluid to work properly but at moderate temperature not very hot not very cold when a membrane solidifies its permeability changes enzyme in it become inactive, however membrane too fluidy cannot support protein function

Membrane Protein and their functions

Different kinds of cells contains different sets of membrane proteins, and the various membranes within a cell each have a unique collection of proteins. More than 50 kinds of proteins have been found in the plasma membrane of red blood cells. The two major populations of membrane protein: Integral proteins and peripheral proteins.

Integral proteins penetrate the hydrophobic interior of the lipid bilayer. The majority are transmembrane proteins, which span the membrane; other integral proteins extend only only partway into the hydrophobic interior. Peripheral protein are present at the periphery of cell membrane  

Proteins and protein complexes perform key functions Although cells interact with their environment through their plasma membranes in many ways, we will focus on six key classes of membrane protein: 1. Transporters. Membranes are very selective, allowing only certain solutes to enter or leave the cell, through either channels or carriers composed of proteins. 2. Enzymes. Cells carry out many chemical reactions on the interior surface of the plasma membrane, using enzymes attached to the membrane. 3. Cell-surface receptors. Membranes are exquisitely sensitive to chemical messages, which are detected by receptor proteins on their surfaces. 4. Cell-surface identity markers. Membranes carry cell-surface markers that identify them to other cells. Most cell types carry their own ID tags, specific combinations of cell-surface proteins and protein complexes such as glycoproteins that are characteristic of that cell type. 5. Cell-to-cell adhesion proteins. Cells use specific proteins to glue themselves to one another. Some act by forming temporary interactions, and others form a morosely bound to the surface of the membrane, they are present at the periphery of membrane.6. Attachments to the cytoskeleton. Surface proteins that interact with other cells are often anchored to the cytoskeleton by linking proteins.

 Passive Transport Across Membranes

Many substances can move in and out of the cell without the cell’s having to expend energy. This type of movement is termed passive transport. Some ions and molecules can pass through the membrane fairly easily and do so because of a concentration gradient—a difference in concentration inside the membrane versus outside. Some substances also move in response to a concentration gradient, but do so through specific protein channels in the membrane. Molecules and ions dissolved in water are in constant random motion. This net movement of these substances from regions of high concentration to regions of lower concentration, a process called diffusion. The major barrier to crossing a biological membrane is the hydrophobic interior that repels polar molecules but not nonpolar molecules. If a concentration difference exists for a nonpolar molecule, it will move across the membrane until the concentration is equal on both sides. At this point, movement in both directions still occurs, but there is no net change in either direction. This includes molecules like O2 and nonpolar organic molecules such as steroid hormones and thus the cell membrane is said to be selectively permeable or semipermeable membrane

Active Transport Across Membranes

Diffusion, facilitated diffusion, and osmosis are passive transport processes that move materials down their concentration gradients, but cells can also actively move substances across a cell membrane up their concentration gradients. This process requires the expenditure of energy, typically from ATP, and is therefore called active transport. Active transport is one of the most important functions of any cell. It enables a cell to take up additional molecules of a substance that is already present in its cytoplasm in concentrations higher than in the extracellular fluid. Active transport also enables a cell to move substances out of its cytoplasm and into the extracellular fluid, despite higher external concentrations. The sodium–potassium pump runs directly on ATP More than one-third of all of the energy expended by an animal cell that is not actively dividing is used in the active transport of sodium (Na+) and potassium (K+) ions. Most animal cells have a low internal concentration of Na+, relative to their surroundings, and a high internal concentration of K+. They maintain these concentration differences by actively pumping Na+ out of the cell and K+ in. The remarkable protein that transports these two ions across the cell membrane is known as the sodium–potassium pump. 

Bulk transport by endocytosis and exocytosis

Bulk transport moves large quantities of substances that cannot pass through the cell membrane. Bulk material enters the cell in vesicles. In endocytosis, the cell membrane surrounds material and pinches off to form a vesicle. In receptor-mediated endocytosis, specific molecules bind to receptors on the cell membrane. Material can leave the cell by exocytosis. In exocytosis, material in a vesicle is discharged when the vesicle fuses with the membrane.

If the material the cell takes in is particulate (made up of discrete particles), such as an organism or some other fragment of organic matter , the process is called phagocytosis (Greek phagein, “to eat,” + cytos, “cell”). If the material the cell takes in is liquid , the process is called pinocytosis (Greek pinein, “to drink”). 

cell membrane is present in all cell before coming to individual existence whether from higher animals to prokaryotes nature is always ready to protect the generations by providing small things like favourable condition or even we can consider cell membrane. Studying biology makes an understanding that how nature has gone deep and creative. Basically it is not just about cramming it is to feel and understand the concepts in biology which we do not even need to visualize it is just present around us and the most important thing is being curious for it. 

I will be back again next time with another topic. Thanks for reading